The conventional image-forming process of silver halide photography includes imagewise exposure of a color photographic silver halide recording material to actinic radiation (such as actinic light), and the eventual manifestation of a useable image by wet photochemical processing of that exposed material. A fundamental step of photochemical processing is the treatment of the material with one or more developing agents to reduce silver halide to silver metal. The useful color image consists of one or more images in organic dyes produced from an oxidized developing agent formed wherever silver halide is reduced to metallic silver.
To obtain useful color images, it is usually necessary to remove all of the silver from the photographic element after color development. This is sometimes known as "desilvering". Removal of silver is generally accomplished by oxidizing the metallic silver, and then dissolving it and undeveloped silver halide with a "solvent" or fixing agent in what is known as a fixing step. Oxidation is achieved with an oxidizing agent, commonly known as a bleaching agent.
One commercially important process intended for color reversal photographic films useful for providing positive color images, can include the following sequence of processing steps: first (or black-and-white) development, washing, reversal reexposure, color development, bleaching, fixing, washing and/or stabilizing. Another useful process has the same steps, but stabilizing is carried out between color development and bleaching. Such conventional steps are described, for example, in U.S. Pat. No. 4,921,779 (Cullinan et al), U.S. Pat. No. 4,975,356 (Cullinan et al), U.S. Pat. No. 5,037,725 (Cullinan et al), U.S. Pat. No. 5,523,195 (Darmon et al) and U.S. Pat. No. 5,552,264 (Cullinan et al).
The most common bleaching agents are complexes of ferric ion and various organic ligands (such as aminopolycarboxylic acids), of which there are hundreds of possibilities, all with varying bleaching activities and biodegradability. Common organic ligands used as part of bleaching agents for color film processing include ethylenediaminetetraacetic acid (EDTA), 1,3-propylenediaminetetraacetic acid (PDTA) and nitrilotriacetic acid (NTA).
U.S. Pat. No. 4,294,914 (Fyson) describes bleaching and bleach-fixing compositions and a processing method using a ferric complex of one of several alkyliminodiacetic acids, which are known to be more biodegradable than other common organic ligands such as EDTA. Other bleaching agents using similar organic ligands are described in U.S. Pat. No. 5,061,608 (Foster et al) in which the bleaching agent is advantageously combined with specific aliphatic carboxylic acids to reduce dye stains. U.S. Pat. No. 5,334,491 (Foster et al) also describes the use of similar biodegradable bleaching agents in combination with specific levels of bromide ion.
In processes utilizing methyliminodiacetic acid (MIDA) as the bleaching agent ligand, iron-MIDA dissociation may occur in the wash bath. This rust formation must be controlled by preventing the dissociation of MIDA from ferric ion. In copending and commonly assigned U.S. Ser. No. 08/795,885, filed Feb. 6, 1997, by Foster, a method of photographic processing is described wherein "anti-rust" agents are included in the bleaching solutions in order to control the dissociation of MIDA and similar ligands from ferric ion. Such agents are defined as organic phosphonic or phosphinic acids or salts thereof and were found to be highly effective. However, the "loading" of photographic bleaching solutions with yet another chemical is not the best way to solve such problems, and can create even further concerns, such as environmental problems.
Fixing is typically carried out after bleaching using a fixing composition that includes one or more fixing agents such as thiosulfate or thiocyanate salts. Both ammonium and sodium salts are known.
For processing color negative photographic films, bleaching compositions containing iron complexes of MIDA and other biodegradable ligands have been used successfully. However, it is has been discovered that such compositions cannot be successfully used to bleach color reversal films because of the less than desirable stability of the color images obtained therefrom. In some of those types of films, the magenta dye forming color coupler leaves a yellow background stain during long term keeping if the films are processed using MIDA-based bleaching compositions. This is believed to occur because of retained iron in the films, which iron may facilitate conversion of a stabilized form of the magenta dye forming coupler to yellow dye.
Thus, while there is a need in the industry to use more biodegradable bleaching compositions to process color reversal films, this need has not yet been met without resulting yellow dye stain in such films. There is a need to provide this advance in the art while reducing the presence of yellow dye stain and retained iron in the processed color reversal films.